The present invention relates to improving interconnect microstructure on semiconductor devices and, more specifically, to selective annealing using a focused laser of specific microscopic areas identified by any technique sensitive to crystal orientation that has the appropriate field of view, such as electron backscatter diffraction (EBSD), as used to map orientation of early-growth grains on the top surface of the device.
The present invention addresses the problem that fine grains having a wide variety of orientations exist on interconnects, leading to lower electromigration (EM) lifetimes and higher resistance values due to electron scattering than would result if grain orientations could be better controlled. Advanced metallization schemes involving higher melting point metals such as cobalt (Co), tungsten (W), Ruthenium (Ru), etc., are expected to struggle to achieve large grain microstructures, thereby again increasing resistance for interconnects. Electromigration is the process by which a metal conductor changes shape under the influence of an electric current flowing through it and which eventually leads to the breaking of the conductor.
Micrograph evaluations, such as exemplarily shown in FIG. 1, have demonstrated that copper (Cu) overburden 102 used as interconnects over patterned features 104 include large grains 106 that do not completely grow through to the underlying patterned features. Also, these evaluations further demonstrate that the pattern feature bottom surface contains more small grains 108 than its top surface 110. Prior art techniques have generally failed to improve grain structure of copper in narrow pitch features.
Although Cu is used in the following discussion to explain the problem identified by the present invention and the solution described herein, the present invention is intended as applicable for any metal used as an overburden to connect to underlying patterned features of a chip. Thus, the present invention addresses the problem of how to locally encourage preferred grain growth through the thickness of a thin film of metal to make optimal contact with an underlying feature.